HTTPbis Working Group M. Belshe
Internet-Draft Twist
Expires: July 26, 2013 R. Peon
Google, Inc
M. Thomson, Ed.
Microsoft
A. Melnikov, Ed.
Isode Ltd
January 22, 2013
Hypertext Transfer Protocol version 2.0
draft-ietf-httpbis-http2-01
Abstract
This document describes an optimised expression of the semantics of
the HTTP protocol. The HTTP/2.0 encapsulation enables more efficient
transfer of resources over HTTP by providing compressed headers,
simultaneous requests, and unsolicited push of resources from server
to client.
This document is an alternative to, but does not obsolete
RFC{http-p1}. The HTTP protocol semantics described in RFC{http-
p2..p7} are unmodified.
Editorial Note (To be removed by RFC Editor)
This draft is a work-in-progress, and does not yet reflect Working
Group consensus.
This draft contains features from the SPDY Protocol as a starting
point, as per the Working Group's charter. Future drafts will add,
remove and change text, based upon the Working Group's decisions.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in Appendix A.1.
Status of This Memo
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Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Document Organization . . . . . . . . . . . . . . . . . . 5
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
2. Starting HTTP/2.0 . . . . . . . . . . . . . . . . . . . . . . 6
2.1. HTTP/2.0 Version Identification . . . . . . . . . . . . . 6
2.2. Starting HTTP/2.0 for "http:" URIs . . . . . . . . . . . . 7
2.3. Starting HTTP/2.0 for "https:" URIs . . . . . . . . . . . 8
3. HTTP/2.0 Framing Layer . . . . . . . . . . . . . . . . . . . . 8
3.1. Session (Connections) . . . . . . . . . . . . . . . . . . 8
3.2. Framing . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Control frames . . . . . . . . . . . . . . . . . . . . 9
3.2.2. Data frames . . . . . . . . . . . . . . . . . . . . . 10
3.3. Streams . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Stream frames . . . . . . . . . . . . . . . . . . . . 11
3.3.2. Stream creation . . . . . . . . . . . . . . . . . . . 11
3.3.3. Stream priority . . . . . . . . . . . . . . . . . . . 12
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3.3.4. Stream headers . . . . . . . . . . . . . . . . . . . . 12
3.3.5. Stream data exchange . . . . . . . . . . . . . . . . . 13
3.3.6. Stream half-close . . . . . . . . . . . . . . . . . . 13
3.3.7. Stream close . . . . . . . . . . . . . . . . . . . . . 13
3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 14
3.4.1. Session Error Handling . . . . . . . . . . . . . . . . 14
3.4.2. Stream Error Handling . . . . . . . . . . . . . . . . 14
3.5. Stream Flow Control . . . . . . . . . . . . . . . . . . . 15
3.5.1. Flow Control Principles . . . . . . . . . . . . . . . 15
3.5.2. Basic Flow Control Algorithm . . . . . . . . . . . . . 16
3.6. Control frame types . . . . . . . . . . . . . . . . . . . 16
3.6.1. SYN_STREAM . . . . . . . . . . . . . . . . . . . . . . 16
3.6.2. SYN_REPLY . . . . . . . . . . . . . . . . . . . . . . 18
3.6.3. RST_STREAM . . . . . . . . . . . . . . . . . . . . . . 19
3.6.4. SETTINGS . . . . . . . . . . . . . . . . . . . . . . . 20
3.6.5. PING . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.6.6. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . 24
3.6.7. HEADERS . . . . . . . . . . . . . . . . . . . . . . . 25
3.6.8. WINDOW_UPDATE . . . . . . . . . . . . . . . . . . . . 26
3.6.9. CREDENTIAL . . . . . . . . . . . . . . . . . . . . . . 28
3.6.10. Name/Value Header Block . . . . . . . . . . . . . . . 30
4. HTTP Layering over HTTP/2.0 . . . . . . . . . . . . . . . . . 36
4.1. Connection Management . . . . . . . . . . . . . . . . . . 36
4.1.1. Use of GOAWAY . . . . . . . . . . . . . . . . . . . . 36
4.2. HTTP Request/Response . . . . . . . . . . . . . . . . . . 37
4.2.1. Request . . . . . . . . . . . . . . . . . . . . . . . 37
4.2.2. Response . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.3. Authentication . . . . . . . . . . . . . . . . . . . . 39
4.3. Server Push Transactions . . . . . . . . . . . . . . . . . 40
4.3.1. Server implementation . . . . . . . . . . . . . . . . 41
4.3.2. Client implementation . . . . . . . . . . . . . . . . 42
5. Design Rationale and Notes . . . . . . . . . . . . . . . . . . 43
5.1. Separation of Framing Layer and Application Layer . . . . 43
5.2. Error handling - Framing Layer . . . . . . . . . . . . . . 43
5.3. One Connection Per Domain . . . . . . . . . . . . . . . . 44
5.4. Fixed vs Variable Length Fields . . . . . . . . . . . . . 44
5.5. Compression Context(s) . . . . . . . . . . . . . . . . . . 45
5.6. Unidirectional streams . . . . . . . . . . . . . . . . . . 45
5.7. Data Compression . . . . . . . . . . . . . . . . . . . . . 45
5.8. Server Push . . . . . . . . . . . . . . . . . . . . . . . 46
6. Security Considerations . . . . . . . . . . . . . . . . . . . 46
6.1. Use of Same-origin constraints . . . . . . . . . . . . . . 46
6.2. HTTP Headers and HTTP/2.0 Headers . . . . . . . . . . . . 46
6.3. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . . 46
6.4. Server Push Implicit Headers . . . . . . . . . . . . . . . 46
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 47
7.1. Long Lived Connections . . . . . . . . . . . . . . . . . . 47
7.2. SETTINGS frame . . . . . . . . . . . . . . . . . . . . . . 47
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8. Requirements Notation . . . . . . . . . . . . . . . . . . . . 47
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
10. Normative References . . . . . . . . . . . . . . . . . . . . . 48
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 49
A.1. Since draft-ietf-httpbis-http2-00 . . . . . . . . . . . . 49
A.2. Since draft-mbelshe-httpbis-spdy-00 . . . . . . . . . . . 49
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1. Introduction
HTTP is a wildly successful protocol. HTTP/1.1 message encapsulation
[HTTP-p1] is optimized for implementation simplicity and
accessibility, not application performance. As such it has several
characteristics that have a negative overall effect on application
performance.
The HTTP/1.1 encapsulation ensures that only one request can be
delivered at a time on a given connection. HTTP/1.1 pipelining,
which is not widely deployed, only partially addresses these
concerns. Clients that need to make multiple requests therefore use
commonly multiple connections to a server or servers in order to
reduce the overall latency of those requests.
Furthermore, HTTP/1.1 headers are represented in an inefficient
fashion, which, in addition to generating more or larger network
packets, can cause the small initial TCP window to fill more quickly
than is ideal. This results in excessive latency where multiple
requests are made on a new TCP connection.
This document defines an optimized mapping of the HTTP semantics to a
TCP connection. This optimization reduces the latency costs of HTTP
by allowing parallel requests on the same connection and by using an
efficient coding for HTTP headers. Prioritization of requests lets
more important requests complete faster, further improving
application performance.
HTTP/2.0 applications have an improved impact on network congestion
due to the use of fewer TCP connections to achieve the same effect.
Fewer TCP connections compete more fairly with other flows. Long-
lived connections are also more able to take better advantage of the
available network capacity, rather than operating in the slow start
phase of TCP.
The HTTP/2.0 encapsulation also enables more efficient processing of
messages by providing efficient message framing. Processing of
headers in HTTP/2.0 messages is more efficient (for entities that
process many messages).
1.1. Document Organization
The HTTP/2.0 Specification is split into three parts: starting
HTTP/2.0 (Section 2), which covers how a HTTP/2.0 is started; a
framing layer (Section 3), which multiplexes a TCP connection into
independent, length-prefixed frames; and an HTTP layer (Section 4),
which specifies the mechanism for overlaying HTTP request/response
pairs on top of the framing layer. While some of the framing layer
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concepts are isolated from the HTTP layer, building a generic framing
layer has not been a goal. The framing layer is tailored to the
needs of the HTTP protocol and server push.
1.2. Definitions
client: The endpoint initiating the HTTP/2.0 session.
connection: A transport-level connection between two endpoints.
endpoint: Either the client or server of a connection.
frame: A header-prefixed sequence of bytes sent over a HTTP/2.0
session.
server: The endpoint which did not initiate the HTTP/2.0 session.
session: A synonym for a connection.
session error: An error on the HTTP/2.0 session.
stream: A bi-directional flow of bytes across a virtual channel
within a HTTP/2.0 session.
stream error: An error on an individual HTTP/2.0 stream.
2. Starting HTTP/2.0
Just as HTTP/1.1 does, HTTP/2.0 uses the "http:" and "https:" URI
schemes. An HTTP/2.0-capable client is therefore required to
discover whether a server (or intermediary) supports HTTP/2.0.
Different discovery mechanisms are defined for "http:" and "https:"
URIs. Discovery for "http:" URIs is described in Section 2.2;
discovery for "https:" URIs is described in Section 2.3.
2.1. HTTP/2.0 Version Identification
HTTP/2.0 is identified in using the string "HTTP/2.0". This
identification is used in the HTTP/1.1 Upgrade header, in the TLS-NPN
[TLSNPN] [[TBD]] field and other places where protocol identification
is required.
[[Editor's Note: please remove the following text prior to the
publication of a final version of this document.]]
Only implementations of the final, published RFC can identify
themselves as "HTTP/2.0". Until such an RFC exists, implementations
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MUST NOT identify themselves using "HTTP/2.0".
Examples and text throughout the rest of this document use "HTTP/2.0"
as a matter of editorial convenience only. Implementations of draft
versions MUST NOT identify using this string.
Implementations of draft versions of the protocol MUST add the
corresponding draft number to the identifier before the separator
('/'). For example, draft-ietf-httpbis-http2-03 is identified using
the string "HTTP-03/2.0".
Non-compatible experiments that are based on these draft versions
MUST include a further identifier. For example, an experimental
implementation of packet mood-based encoding based on
draft-ietf-httpbis-http2-07 might identify itself as "HTTP-07-
emo/2.0". Note that any label MUST conform with the "token" syntax
defined in Section 3.2.4 of [HTTP-p1]. Experimenters are encouraged
to coordinate their experiments on the ietf-http-wg@w3.org mailing
list.
2.2. Starting HTTP/2.0 for "http:" URIs
A client that makes a request to an "http:" URI without prior
knowledge about support for HTTP/2.0 uses the HTTP Upgrade mechanism
[HTTP-p2]. The client makes an HTTP/1.1 request that includes an
Upgrade header field identifying HTTP/2.0.
For example:
GET /default.htm HTTP/1.1
Host: server.example.com
Connection: Upgrade
Upgrade: HTTP/2.0
A server that does not support HTTP/2.0 can respond to the request as
though the Upgrade header field were absent:
HTTP/1.1 200 OK
Content-length: 243
Content-type: text/html
...
A server that supports HTTP/2.0 can accept the upgrade with a 101
(Switching Protocols) status code. After the empty line that
terminates the 101 response, the server can begin sending HTTP/2.0
frames. These frames MUST include a response to the request that
initiated the Upgrade.
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HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: HTTP/2.0
[ HTTP/2.0 frames ...
A client can learn that a particular server supports HTTP/2.0 by
other means. A client MAY immediately send HTTP/2.0 frames to a
server that is known to support HTTP/2.0. [[Open Issue: This is not
definite. We may yet choose to perform negotiation for every
connection. Reasons include intermediaries; phased upgrade of load-
balanced server farms; etc...]] [[Open Issue: We need to enumerate
the ways that clients can learn of HTTP/2.0 support.]]
2.3. Starting HTTP/2.0 for "https:" URIs
[[TBD, maybe NPN]]
3. HTTP/2.0 Framing Layer
3.1. Session (Connections)
The HTTP/2.0 framing layer (or "session") runs atop a reliable
transport layer such as TCP [RFC0793]. The client is the TCP
connection initiator. HTTP/2.0 connections are persistent
connections.
For best performance, it is expected that clients will not close open
connections until the user navigates away from all web pages
referencing a connection, or until the server closes the connection.
Servers are encouraged to leave connections open for as long as
possible, but can terminate idle connections if necessary. When
either endpoint closes the transport-level connection, it MUST first
send a GOAWAY (Section 3.6.6) frame so that the endpoints can
reliably determine if requests finished before the close.
3.2. Framing
Once the connection is established, clients and servers exchange
framed messages. There are two types of frames: control frames
(Section 3.2.1) and data frames (Section 3.2.2). Frames always have
a common header which is 8 bytes in length.
The first bit is a control bit indicating whether a frame is a
control frame or data frame. Control frames carry a version number,
a frame type, flags, and a length. Data frames contain the stream
ID, flags, and the length for the payload carried after the common
header. The simple header is designed to make reading and writing of
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frames easy.
All integer values, including length, version, and type, are in
network byte order. HTTP/2.0 does not enforce alignment of types in
dynamically sized frames.
3.2.1. Control frames
+----------------------------------+
|C| Version(15bits) | Type(16bits) |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Data |
+----------------------------------+
Control bit: The 'C' bit is a single bit indicating if this is a
control message. For control frames this value is always 1.
Version: The version number of the HTTP/2.0 protocol. This document
describes HTTP/2.0 version 3.
Type: The type of control frame. See Control Frames for the complete
list of control frames.
Flags: Flags related to this frame. Flags for control frames and
data frames are different.
Length: An unsigned 24-bit value representing the number of bytes
after the length field.
Data: data associated with this control frame. The format and length
of this data is controlled by the control frame type.
Control frame processing requirements:
Note that full length control frames (16MB) can be large for
implementations running on resource-limited hardware. In such
cases, implementations MAY limit the maximum length frame
supported. However, all implementations MUST be able to receive
control frames of at least 8192 octets in length.
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3.2.2. Data frames
+----------------------------------+
|C| Stream-ID (31bits) |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Data |
+----------------------------------+
Control bit: For data frames this value is always 0.
Stream-ID: A 31-bit value identifying the stream.
Flags: Flags related to this frame. Valid flags are:
0x01 = FLAG_FIN - signifies that this frame represents the last
frame to be transmitted on this stream. See Stream Close
(Section 3.3.7) below.
0x02 = FLAG_COMPRESS - indicates that the data in this frame has
been compressed.
Length: An unsigned 24-bit value representing the number of bytes
after the length field. The total size of a data frame is 8 bytes +
length. It is valid to have a zero-length data frame.
Data: The variable-length data payload; the length was defined in the
length field.
Data frame processing requirements:
If an endpoint receives a data frame for a stream-id which is not
open and the endpoint has not sent a GOAWAY (Section 3.6.6) frame,
it MUST send issue a stream error (Section 3.4.2) with the error
code INVALID_STREAM for the stream-id.
If the endpoint which created the stream receives a data frame
before receiving a SYN_REPLY on that stream, it is a protocol
error, and the recipient MUST issue a stream error (Section 3.4.2)
with the status code PROTOCOL_ERROR for the stream-id.
Implementors note: If an endpoint receives multiple data frames
for invalid stream-ids, it MAY close the session.
All HTTP/2.0 endpoints MUST accept compressed data frames.
Compression of data frames is always done using zlib compression.
Each stream initializes and uses its own compression context
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dedicated to use within that stream. Endpoints are encouraged to
use application level compression rather than HTTP/2.0 stream
level compression.
Each HTTP/2.0 stream sending compressed frames creates its own
zlib context for that stream, and these compression contexts MUST
be distinct from the compression contexts used with SYN_STREAM/
SYN_REPLY/HEADER compression. (Thus, if both endpoints of a
stream are compressing data on the stream, there will be two zlib
contexts, one for sending and one for receiving).
3.3. Streams
Streams are independent sequences of bi-directional data divided into
frames with several properties:
Streams may be created by either the client or server.
Streams optionally carry a set of name/value header pairs.
Streams can concurrently send data interleaved with other streams.
Streams may be cancelled.
3.3.1. Stream frames
HTTP/2.0 defines 3 control frames to manage the lifecycle of a
stream:
SYN_STREAM - Open a new stream
SYN_REPLY - Remote acknowledgement of a new, open stream
RST_STREAM - Close a stream
3.3.2. Stream creation
A stream is created by sending a control frame with the type set to
SYN_STREAM (Section 3.6.1). If the server is initiating the stream,
the Stream-ID must be even. If the client is initiating the stream,
the Stream-ID must be odd. 0 is not a valid Stream-ID. Stream-IDs
from each side of the connection must increase monotonically as new
streams are created. E.g. Stream 2 may be created after stream 3,
but stream 7 must not be created after stream 9. Stream IDs do not
wrap: when a client or server cannot create a new stream id without
exceeding a 31 bit value, it MUST NOT create a new stream.
The stream-id MUST increase with each new stream. If an endpoint
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receives a SYN_STREAM with a stream id which is less than any
previously received SYN_STREAM, it MUST issue a session error
(Section 3.4.1) with the status PROTOCOL_ERROR.
It is a protocol error to send two SYN_STREAMs with the same
stream-id. If a recipient receives a second SYN_STREAM for the same
stream, it MUST issue a stream error (Section 3.4.2) with the status
code PROTOCOL_ERROR.
Upon receipt of a SYN_STREAM, the recipient can reject the stream by
sending a stream error (Section 3.4.2) with the error code
REFUSED_STREAM. Note, however, that the creating endpoint may have
already sent additional frames for that stream which cannot be
immediately stopped.
Once the stream is created, the creator may immediately send HEADERS
or DATA frames for that stream, without needing to wait for the
recipient to acknowledge.
3.3.2.1. Unidirectional streams
When an endpoint creates a stream with the FLAG_UNIDIRECTIONAL flag
set, it creates a unidirectional stream which the creating endpoint
can use to send frames, but the receiving endpoint cannot. The
receiving endpoint is implicitly already in the half-closed
(Section 3.3.6) state.
3.3.2.2. Bidirectional streams
SYN_STREAM frames which do not use the FLAG_UNIDIRECTIONAL flag are
bidirectional streams. Both endpoints can send data on a bi-
directional stream.
3.3.3. Stream priority
The creator of a stream assigns a priority for that stream. Priority
is represented as an integer from 0 to 7. 0 represents the highest
priority and 7 represents the lowest priority.
The sender and recipient SHOULD use best-effort to process streams in
the order of highest priority to lowest priority.
3.3.4. Stream headers
Streams carry optional sets of name/value pair headers which carry
metadata about the stream. After the stream has been created, and as
long as the sender is not closed (Section 3.3.7) or half-closed
(Section 3.3.6), each side may send HEADERS frame(s) containing the
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header data. Header data can be sent in multiple HEADERS frames, and
HEADERS frames may be interleaved with data frames.
3.3.5. Stream data exchange
Once a stream is created, it can be used to send arbitrary amounts of
data. Generally this means that a series of data frames will be sent
on the stream until a frame containing the FLAG_FIN flag is set. The
FLAG_FIN can be set on a SYN_STREAM (Section 3.6.1), SYN_REPLY
(Section 3.6.2), HEADERS (Section 3.6.7) or a DATA (Section 3.2.2)
frame. Once the FLAG_FIN has been sent, the stream is considered to
be half-closed.
3.3.6. Stream half-close
When one side of the stream sends a frame with the FLAG_FIN flag set,
the stream is half-closed from that endpoint. The sender of the
FLAG_FIN MUST NOT send further frames on that stream. When both
sides have half-closed, the stream is closed.
If an endpoint receives a data frame after the stream is half-closed
from the sender (e.g. the endpoint has already received a prior frame
for the stream with the FIN flag set), it MUST send a RST_STREAM to
the sender with the status STREAM_ALREADY_CLOSED.
3.3.7. Stream close
There are 3 ways that streams can be terminated:
Normal termination: Normal stream termination occurs when both
sender and recipient have half-closed the stream by sending a
FLAG_FIN.
Abrupt termination: Either the client or server can send a
RST_STREAM control frame at any time. A RST_STREAM contains an
error code to indicate the reason for failure. When a RST_STREAM
is sent from the stream originator, it indicates a failure to
complete the stream and that no further data will be sent on the
stream. When a RST_STREAM is sent from the stream recipient, the
sender, upon receipt, should stop sending any data on the stream.
The stream recipient should be aware that there is a race between
data already in transit from the sender and the time the
RST_STREAM is received. See Stream Error Handling (Section 3.4.2)
TCP connection teardown: If the TCP connection is torn down while
un-closed streams exist, then the endpoint must assume that the
stream was abnormally interrupted and may be incomplete.
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If an endpoint receives a data frame after the stream is closed, it
must send a RST_STREAM to the sender with the status PROTOCOL_ERROR.
3.4. Error Handling
The HTTP/2.0 framing layer has only two types of errors, and they are
always handled consistently. Any reference in this specification to
"issue a session error" refers to Section 3.4.1. Any reference to
"issue a stream error" refers to Section 3.4.2.
3.4.1. Session Error Handling
A session error is any error which prevents further processing of the
framing layer or which corrupts the session compression state. When
a session error occurs, the endpoint encountering the error MUST
first send a GOAWAY (Section 3.6.6) frame with the stream id of most
recently received stream from the remote endpoint, and the error code
for why the session is terminating. After sending the GOAWAY frame,
the endpoint MUST close the TCP connection.
Note that the session compression state is dependent upon both
endpoints always processing all compressed data. If an endpoint
partially processes a frame containing compressed data without
updating compression state properly, future control frames which use
compression will be always be errored. Implementations SHOULD always
try to process compressed data so that errors which could be handled
as stream errors do not become session errors.
Note that because this GOAWAY is sent during a session error case, it
is possible that the GOAWAY will not be reliably received by the
receiving endpoint. It is a best-effort attempt to communicate with
the remote about why the session is going down.
3.4.2. Stream Error Handling
A stream error is an error related to a specific stream-id which does
not affect processing of other streams at the framing layer. Upon a
stream error, the endpoint MUST send a RST_STREAM (Section 3.6.3)
frame which contains the stream id of the stream where the error
occurred and the error status which caused the error. After sending
the RST_STREAM, the stream is closed to the sending endpoint. After
sending the RST_STREAM, if the sender receives any frames other than
a RST_STREAM for that stream id, it will result in sending additional
RST_STREAM frames. An endpoint MUST NOT send a RST_STREAM in
response to an RST_STREAM, as doing so would lead to RST_STREAM
loops. Sending a RST_STREAM does not cause the HTTP/2.0 session to
be closed.
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If an endpoint has multiple RST_STREAM frames to send in succession
for the same stream-id and the same error code, it MAY coalesce them
into a single RST_STREAM frame. (This can happen if a stream is
closed, but the remote sends multiple data frames. There is no
reason to send a RST_STREAM for each frame in succession).
3.5. Stream Flow Control
Multiplexing streams introduces contention for access to the shared
TCP connection. Stream contention can result in streams being
blocked by other streams. A flow control scheme ensures that streams
do not destructively interfere with other streams on the same TCP
connection.
3.5.1. Flow Control Principles
Experience with TCP congestion control has shown that algorithms can
evolve over time to become more sophisticated without requiring
protocol changes. TCP congestion control and its evolution is
clearly different from HTTP/2.0 flow control, though the evolution of
TCP congestion control algorithms shows that a similar approach could
be feasible for HTTP/2.0 flow control.
HTTP/2.0 stream flow control aims to allow for future improvements to
flow control algorithms without requiring protocol changes. The
following principles guide the HTTP/2.0 design:
1. Flow control is hop-by-hop, not end-to-end.
2. Flow control is based on window update messages. Receivers
advertise how many octets they are prepared to receive on a
stream. This is a credit-based scheme.
3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it
desires for each stream [[TBD: ... and for the overall
connection]]. A sender MUST respect flow control limits imposed
by a receiver. Clients, servers and intermediaries all
independently advertise their flow control preferences as a
receiver and abide by the flow control limits set by their peer
when sending.
4. Flow control can be disabled by a receiver. A receiver can
choose to either disable flow control, or to declare an infinite
flow control limit. [[TBD: determine whether just one mechanism
is sufficient, and then which alternative]]
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5. HTTP/2.0 standardizes only the format of the window update
message (Section 3.6.8). This does not stipulate how a receiver
decides when to send this message or the value that it sends.
Nor does it specify how a sender chooses to send packets.
Implementations are able to select any algorithm that suits their
needs. An example flow control algorithm is described in
Section 3.5.2.
Implementations are also responsible for managing how requests and
responses are sent based on priority; choosing how to avoid head of
line blocking for requests; and managing the creation of new streams.
Algorithm choices for these could interact with any flow control
algorithm.
3.5.2. Basic Flow Control Algorithm
This section describes a basic flow control algorithm. This
algorithm is provided as an example, implementations can use any
algorithm that complies with flow control requirements.
[[Algorithm TBD]]
3.6. Control frame types
3.6.1. SYN_STREAM
The SYN_STREAM control frame allows the sender to asynchronously
create a stream between the endpoints. See Stream Creation
(Section 3.3.2)
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+------------------------------------+
|1| version | 1 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
|X| Associated-To-Stream-ID (31bits) |
+------------------------------------+
| Pri|Unused | Slot | |
+-------------------+ |
| Number of Name/Value pairs (int32) |
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CREDENTIAL vector of the client certificate to be used for this
request. see CREDENTIAL frame (Section 3.6.9). The value 0 means no
client certificate should be associated with this stream.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
If an endpoint receives a SYN_STREAM which is larger than the
implementation supports, it MAY send a RST_STREAM with error code
FRAME_TOO_LARGE. All implementations MUST support the minimum size
limits defined in the Control Frames section (Section 3.2.1).
3.6.2. SYN_REPLY
SYN_REPLY indicates the acceptance of a stream creation by the
recipient of a SYN_STREAM frame.
+------------------------------------+
|1| version | 2 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
| Number of Name/Value pairs (int32) |
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If an endpoint receives multiple SYN_REPLY frames for the same active
stream ID, it MUST issue a stream error (Section 3.4.2) with the
error code STREAM_IN_USE.
Name/Value Header Block: A set of name/value pairs carried as part of
the SYN_STREAM. see Name/Value Header Block (Section 3.6.10).
If an endpoint receives a SYN_REPLY which is larger than the
implementation supports, it MAY send a RST_STREAM with error code
FRAME_TOO_LARGE. All implementations MUST support the minimum size
limits defined in the Control Frames section (Section 3.2.1).
3.6.3. RST_STREAM
The RST_STREAM frame allows for abnormal termination of a stream.
When sent by the creator of a stream, it indicates the creator wishes
to cancel the stream. When sent by the recipient of a stream, it
indicates an error or that the recipient did not want to accept the
stream, so the stream should be closed.
+----------------------------------+
|1| version | 3 |
+----------------------------------+
| Flags (8) | 8 |
+----------------------------------+
|X| Stream-ID (31bits) |
+----------------------------------+
| Status code |
+----------------------------------+
Flags: Flags related to this frame. RST_STREAM does not define any
flags. This value must be 0.
Length: An unsigned 24-bit value representing the number of bytes
after the length field. For RST_STREAM control frames, this value is
always 8.
Stream-ID: The 31-bit identifier for this stream.
Status code: (32 bits) An indicator for why the stream is being
terminated.The following status codes are defined:
1 - PROTOCOL_ERROR. This is a generic error, and should only be
used if a more specific error is not available.
2 - INVALID_STREAM. This is returned when a frame is received for
a stream which is not active.
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3 - REFUSED_STREAM. Indicates that the stream was refused before
any processing has been done on the stream.
4 - UNSUPPORTED_VERSION. Indicates that the recipient of a stream
does not support the HTTP/2.0 version requested.
5 - CANCEL. Used by the creator of a stream to indicate that the
stream is no longer needed.
6 - INTERNAL_ERROR. This is a generic error which can be used
when the implementation has internally failed, not due to anything
in the protocol.
7 - FLOW_CONTROL_ERROR. The endpoint detected that its peer
violated the flow control protocol.
8 - STREAM_IN_USE. The endpoint received a SYN_REPLY for a stream
already open.
9 - STREAM_ALREADY_CLOSED. The endpoint received a data or
SYN_REPLY frame for a stream which is half closed.
10 - INVALID_CREDENTIALS. The server received a request for a
resource whose origin does not have valid credentials in the
client certificate vector.
11 - FRAME_TOO_LARGE. The endpoint received a frame which this
implementation could not support. If FRAME_TOO_LARGE is sent for
a SYN_STREAM, HEADERS, or SYN_REPLY frame without fully processing
the compressed portion of those frames, then the compression state
will be out-of-sync with the other endpoint. In this case,
senders of FRAME_TOO_LARGE MUST close the session.
Note: 0 is not a valid status code for a RST_STREAM.
After receiving a RST_STREAM on a stream, the recipient must not send
additional frames for that stream, and the stream moves into the
closed state.
3.6.4. SETTINGS
A SETTINGS frame contains a set of id/value pairs for communicating
configuration data about how the two endpoints may communicate.
SETTINGS frames can be sent at any time by either endpoint, are
optionally sent, and are fully asynchronous. When the server is the
sender, the sender can request that configuration data be persisted
by the client across HTTP/2.0 sessions and returned to the server in
future communications.
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Persistence of SETTINGS ID/Value pairs is done on a per origin/IP
pair (the "origin" is the set of scheme, host, and port from the URI.
See [RFC6454]). That is, when a client connects to a server, and the
server persists settings within the client, the client SHOULD return
the persisted settings on future connections to the same origin AND
IP address and TCP port. Clients MUST NOT request servers to use the
persistence features of the SETTINGS frames, and servers MUST ignore
persistence related flags sent by a client.
+----------------------------------+
|1| version | 4 |
+----------------------------------+
| Flags (8) | Length (24 bits) |
+----------------------------------+
| Number of entries |
+----------------------------------+
| ID/Value Pairs |
| ... |
Control bit: The control bit is always 1 for this message.
Version: The HTTP/2.0 version number.
Type: The message type for a SETTINGS message is 4.
Flags: FLAG_SETTINGS_CLEAR_SETTINGS (0x1): When set, the client
should clear any previously persisted SETTINGS ID/Value pairs. If
this frame contains ID/Value pairs with the
FLAG_SETTINGS_PERSIST_VALUE set, then the client will first clear its
existing, persisted settings, and then persist the values with the
flag set which are contained within this frame. Because persistence
is only implemented on the client, this flag can only be used when
the sender is the server.
Length: An unsigned 24-bit value representing the number of bytes
after the length field. The total size of a SETTINGS frame is 8
bytes + length.
Number of entries: A 32-bit value representing the number of ID/value
pairs in this message.
ID: A 32-bit ID number, comprised of 8 bits of flags and 24 bits of
unique ID.
ID.flags:
FLAG_SETTINGS_PERSIST_VALUE (0x1): When set, the sender of this
SETTINGS frame is requesting that the recipient persist the ID/
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Value and return it in future SETTINGS frames sent from the
sender to this recipient. Because persistence is only
implemented on the client, this flag is only sent by the
server.
FLAG_SETTINGS_PERSISTED (0x2): When set, the sender is
notifying the recipient that this ID/Value pair was previously
sent to the sender by the recipient with the
FLAG_SETTINGS_PERSIST_VALUE, and the sender is returning it.
Because persistence is only implemented on the client, this
flag is only sent by the client.
Defined IDs:
1 - SETTINGS_UPLOAD_BANDWIDTH allows the sender to send its
expected upload bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes
per second that the sender predicts as an expected maximum
upload channel capacity.
2 - SETTINGS_DOWNLOAD_BANDWIDTH allows the sender to send its
expected download bandwidth on this channel. This number is an
estimate. The value should be the integral number of kilobytes
per second that the sender predicts as an expected maximum
download channel capacity.
3 - SETTINGS_ROUND_TRIP_TIME allows the sender to send its
expected round-trip-time on this channel. The round trip time
is defined as the minimum amount of time to send a control
frame from this client to the remote and receive a response.
The value is represented in milliseconds.
4 - SETTINGS_MAX_CONCURRENT_STREAMS allows the sender to inform
the remote endpoint the maximum number of concurrent streams
which it will allow. By default there is no limit. For
implementors it is recommended that this value be no smaller
than 100.
5 - SETTINGS_CURRENT_CWND allows the sender to inform the
remote endpoint of the current TCP CWND value.
6 - SETTINGS_DOWNLOAD_RETRANS_RATE allows the sender to inform
the remote endpoint the retransmission rate (bytes
retransmitted / total bytes transmitted).
7 - SETTINGS_INITIAL_WINDOW_SIZE allows the sender to inform
the remote endpoint the initial window size (in bytes) for new
streams.
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8 - SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE allows the server
to inform the client of the new size of the client certificate
vector.
Value: A 32-bit value.
The message is intentionally extensible for future information which
may improve client-server communications. The sender does not need
to send every type of ID/value. It must only send those for which it
has accurate values to convey. When multiple ID/value pairs are
sent, they should be sent in order of lowest id to highest id. A
single SETTINGS frame MUST not contain multiple values for the same
ID. If the recipient of a SETTINGS frame discovers multiple values
for the same ID, it MUST ignore all values except the first one.
A server may send multiple SETTINGS frames containing different ID/
Value pairs. When the same ID/Value is sent twice, the most recent
value overrides any previously sent values. If the server sends IDs
1, 2, and 3 with the FLAG_SETTINGS_PERSIST_VALUE in a first SETTINGS
frame, and then sends IDs 4 and 5 with the
FLAG_SETTINGS_PERSIST_VALUE, when the client returns the persisted
state on its next SETTINGS frame, it SHOULD send all 5 settings (1,
2, 3, 4, and 5 in this example) to the server.
3.6.5. PING
The PING control frame is a mechanism for measuring a minimal round-
trip time from the sender. It can be sent from the client or the
server. Recipients of a PING frame should send an identical frame to
the sender as soon as possible (if there is other pending data
waiting to be sent, PING should take highest priority). Each ping
sent by a sender should use a unique ID.
+----------------------------------+
|1| version | 6 |
+----------------------------------+
| 0 (flags) | 4 (length) |
+----------------------------------|
| 32-bit ID |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The HTTP/2.0 version number.
Type: The message type for a PING message is 6.
Length: This frame is always 4 bytes long.
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ID: A unique ID for this ping, represented as an unsigned 32 bit
value. When the client initiates a ping, it must use an odd numbered
ID. When the server initiates a ping, it must use an even numbered
ping. Use of odd/even IDs is required in order to avoid accidental
looping on PINGs (where each side initiates an identical PING at the
same time).
Note: If a sender uses all possible PING ids (e.g. has sent all 2^31
possible IDs), it can wrap and start re-using IDs.
If a server receives an even numbered PING which it did not initiate,
it must ignore the PING. If a client receives an odd numbered PING
which it did not initiate, it must ignore the PING.
3.6.6. GOAWAY
The GOAWAY control frame is a mechanism to tell the remote side of
the connection to stop creating streams on this session. It can be
sent from the client or the server. Once sent, the sender will not
respond to any new SYN_STREAMs on this session. Recipients of a
GOAWAY frame must not send additional streams on this session,
although a new session can be established for new streams. The
purpose of this message is to allow an endpoint to gracefully stop
accepting new streams (perhaps for a reboot or maintenance), while
still finishing processing of previously established streams.
There is an inherent race condition between an endpoint sending
SYN_STREAMs and the remote sending a GOAWAY message. To deal with
this case, the GOAWAY contains a last-stream-id indicating the
stream-id of the last stream which was created on the sending
endpoint in this session. If the receiver of the GOAWAY sent new
SYN_STREAMs for sessions after this last-stream-id, they were not
processed by the server and the receiver may treat the stream as
though it had never been created at all (hence the receiver may want
to re-create the stream later on a new session).
Endpoints should always send a GOAWAY message before closing a
connection so that the remote can know whether a stream has been
partially processed or not. (For example, if an HTTP client sends a
POST at the same time that a server closes a connection, the client
cannot know if the server started to process that POST request if the
server does not send a GOAWAY frame to indicate where it stopped
working).
After sending a GOAWAY message, the sender must ignore all SYN_STREAM
frames for new streams.
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+----------------------------------+
|1| version | 7 |
+----------------------------------+
| 0 (flags) | 8 (length) |
+----------------------------------|
|X| Last-good-stream-ID (31 bits) |
+----------------------------------+
| Status code |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The HTTP/2.0 version number.
Type: The message type for a GOAWAY message is 7.
Length: This frame is always 8 bytes long.
Last-good-stream-Id: The last stream id which was replied to (with
either a SYN_REPLY or RST_STREAM) by the sender of the GOAWAY
message. If no streams were replied to, this value MUST be 0.
Status: The reason for closing the session.
0 - OK. This is a normal session teardown.
1 - PROTOCOL_ERROR. This is a generic error, and should only be
used if a more specific error is not available.
2 - INTERNAL_ERROR. This is a generic error which can be used
when the implementation has internally failed, not due to anything
in the protocol.
3.6.7. HEADERS
The HEADERS frame augments a stream with additional headers. It may
be optionally sent on an existing stream at any time. Specific
application of the headers in this frame is application-dependent.
The name/value header block within this frame is compressed.
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+------------------------------------+
|1| version | 8 |
+------------------------------------+
| Flags (8) | Length (24 bits) |
+------------------------------------+
|X| Stream-ID (31bits) |
+------------------------------------+
| Number of Name/Value pairs (int32) |
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Flow control in HTTP/2.0 is implemented by a data transfer window
kept by the sender of each stream. The data transfer window is a
simple uint32 that indicates how many bytes of data the sender can
transmit. After a stream is created, but before any data frames have
been transmitted, the sender begins with the initial window size.
This window size is a measure of the buffering capability of the
recipient. The sender must not send a data frame with data length
greater than the transfer window size. After sending each data
frame, the sender decrements its transfer window size by the amount
of data transmitted. When the window size becomes less than or equal
to 0, the sender must pause transmitting data frames. At the other
end of the stream, the recipient sends a WINDOW_UPDATE control back
to notify the sender that it has consumed some data and freed up
buffer space to receive more data.
+----------------------------------+
|1| version | 9 |
+----------------------------------+
| 0 (flags) | 8 (length) |
+----------------------------------+
|X| Stream-ID (31-bits) |
+----------------------------------+
|X| Delta-Window-Size (31-bits) |
+----------------------------------+
Control bit: The control bit is always 1 for this message.
Version: The HTTP/2.0 version number.
Type: The message type for a WINDOW_UPDATE message is 9.
Length: The length field is always 8 for this frame (there are 8
bytes after the length field).
Stream-ID: The stream ID that this WINDOW_UPDATE control frame is
for.
Delta-Window-Size: The additional number of bytes that the sender can
transmit in addition to existing remaining window size. The legal
range for this field is 1 to 2^31 - 1 (0x7fffffff) bytes.
The window size as kept by the sender must never exceed 2^31
(although it can become negative in one special case). If a sender
receives a WINDOW_UPDATE that causes the its window size to exceed
this limit, it must send RST_STREAM with status code
FLOW_CONTROL_ERROR to terminate the stream.
When a HTTP/2.0 connection is first established, the default initial
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window size for all streams is 64KB. An endpoint can use the
SETTINGS control frame to adjust the initial window size for the
connection. That is, its peer can start out using the 64KB default
initial window size when sending data frames before receiving the
SETTINGS. Because SETTINGS is asynchronous, there may be a race
condition if the recipient wants to decrease the initial window size,
but its peer immediately sends 64KB on the creation of a new
connection, before waiting for the SETTINGS to arrive. This is one
case where the window size kept by the sender will become negative.
Once the sender detects this condition, it must stop sending data
frames and wait for the recipient to catch up. The recipient has two
choices:
immediately send RST_STREAM with FLOW_CONTROL_ERROR status code.
allow the head of line blocking (as there is only one stream for
the session and the amount of data in flight is bounded by the
default initial window size), and send WINDOW_UPDATE as it
consumes data.
In the case of option 2, both sides must compute the window size
based on the initial window size in the SETTINGS. For example, if
the recipient sets the initial window size to be 16KB, and the sender
sends 64KB immediately on connection establishment, the sender will
discover its window size is -48KB on receipt of the SETTINGS. As the
recipient consumes the first 16KB, it must send a WINDOW_UPDATE of
16KB back to the sender. This interaction continues until the
sender's window size becomes positive again, and it can resume
transmitting data frames.
After the recipient reads in a data frame with FLAG_FIN that marks
the end of the data stream, it should not send WINDOW_UPDATE frames
as it consumes the last data frame. A sender should ignore all the
WINDOW_UPDATE frames associated with the stream after it send the
last frame for the stream.
The data frames from the sender and the WINDOW_UPDATE frames from the
recipient are completely asynchronous with respect to each other.
This property allows a recipient to aggressively update the window
size kept by the sender to prevent the stream from stalling.
3.6.9. CREDENTIAL
The CREDENTIAL control frame is used by the client to send additional
client certificates to the server. A HTTP/2.0 client may decide to
send requests for resources from different origins on the same
HTTP/2.0 session if it decides that that server handles both origins.
For example if the IP address associated with both hostnames matches
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and the SSL server certificate presented in the initial handshake is
valid for both hostnames. However, because the SSL connection can
contain at most one client certificate, the client needs a mechanism
to send additional client certificates to the server.
The server is required to maintain a vector of client certificates
associated with a HTTP/2.0 session. When the client needs to send a
client certificate to the server, it will send a CREDENTIAL frame
that specifies the index of the slot in which to store the
certificate as well as proof that the client posesses the
corresponding private key. The initial size of this vector must be
8. If the client provides a client certificate during the first TLS
handshake, the contents of this certificate must be copied into the
first slot (index 1) in the CREDENTIAL vector, though it may be
overwritten by subsequent CREDENTIAL frames. The server must
exclusively use the CREDENTIAL vector when evaluating the client
certificates associated with an origin. The server may change the
size of this vector by sending a SETTINGS frame with the setting
SETTINGS_CLIENT_CERTIFICATE_VECTOR_SIZE value specified. In the
event that the new size is smaller than the current size, truncation
occurs preserving lower-index slots as possible.
TLS renegotiation with client authentication is incompatible with
HTTP/2.0 given the multiplexed nature of HTTP/2.0. Specifically,
imagine that the client has 2 requests outstanding to the server for
two different pages (in different tabs). When the renegotiation +
client certificate request comes in, the browser is unable to
determine which resource triggered the client certificate request, in
order to prompt the user accordingly.
+----------------------------------+
|1|000000000000001|0000000000001011|
+----------------------------------+
| flags (8) | Length (24 bits) |
+----------------------------------+
| Slot (16 bits) | |
+-----------------+ |
| Proof Length (32 bits) |
+----------------------------------+
| Proof |
+----------------------------------+
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stored at this index, it will be overwritten. The index is one
based, not zero based; zero is an invalid slot index.
Proof: Cryptographic proof that the client has possession of the
private key associated with the certificate. The format is a TLS
digitally-signed element ([RFC5246], Section 4.7). The signature
algorithm must be the same as that used in the CertificateVerify
message. However, since the MD5+SHA1 signature type used in TLS 1.0
connections can not be correctly encoded in a digitally-signed
element, SHA1 must be used when MD5+SHA1 was used in the SSL
connection. The signature is calculated over a 32 byte TLS extractor
value (http://tools.ietf.org/html/rfc5705) with a label of "EXPORTER
HTTP/2.0 certificate proof" using the empty string as context.
ForRSA certificates the signature would be a PKCS#1 v1.5 signature.
For ECDSA, it would be an ECDSA-Sig-Value
(http://tools.ietf.org/html/rfc5480#appendix-A). For a 1024-bit RSA
key, the CREDENTIAL message would be ~500 bytes.
Certificate: The certificate chain, starting with the leaf
certificate. Each certificate must be encoded as a 32 bit length,
followed by a DER encoded certificate. The certificate must be of
the same type (RSA, ECDSA, etc) as the client certificate associated
with the SSL connection.
If the server receives a request for a resource with unacceptable
credential (either missing or invalid), it must reply with a
RST_STREAM frame with the status code INVALID_CREDENTIALS. Upon
receipt of a RST_STREAM frame with INVALID_CREDENTIALS, the client
should initiate a new stream directly to the requested origin and
resend the request. Note, HTTP/2.0 does not allow the server to
request different client authentication for different resources in
the same origin.
If the server receives an invalid CREDENTIAL frame, it MUST respond
with a GOAWAY frame and shutdown the session.
3.6.10. Name/Value Header Block
The Name/Value Header Block is found in the SYN_STREAM, SYN_REPLY and
HEADERS control frames, and shares a common format:
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+------------------------------------+
| Number of Name/Value pairs (int32) |
+------------------------------------+
| Length of name (int32) |
+------------------------------------+
| Name (string) |
+------------------------------------+
| Length of value (int32) |
+------------------------------------+
| Value (string) |
+------------------------------------+
| (repeats) |
Number of Name/Value pairs: The number of repeating name/value pairs
following this field.
List of Name/Value pairs:
Length of Name: a 32-bit value containing the number of octets in
the name field. Note that in practice, this length must not
exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
Name: 0 or more octets, 8-bit sequences of data, excluding 0.
Length of Value: a 32-bit value containing the number of octets in
the value field. Note that in practice, this length must not
exceed 2^24, as that is the maximum size of a HTTP/2.0 frame.
Value: 0 or more octets, 8-bit sequences of data, excluding 0.
Each header name must have at least one value. Header names are
encoded using the US-ASCII character set [ASCII] and must be all
lower case. The length of each name must be greater than zero. A
recipient of a zero-length name MUST issue a stream error
(Section 3.4.2) with the status code PROTOCOL_ERROR for the
stream-id.
Duplicate header names are not allowed. To send two identically
named headers, send a header with two values, where the values are
separated by a single NUL (0) byte. A header value can either be
empty (e.g. the length is zero) or it can contain multiple, NUL-
separated values, each with length greater than zero. The value
never starts nor ends with a NUL character. Recipients of illegal
value fields MUST issue a stream error (Section 3.4.2) with the
status code PROTOCOL_ERROR for the stream-id.
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Internet-Draft HTTP/2.0 January 2013
3.6.10.1. Compression
The Name/Value Header Block is a section of the SYN_STREAM,
SYN_REPLY, and HEADERS frames used to carry header meta-data. This
block is always compressed using zlib compression. Within this
specification, any reference to 'zlib' is referring to the ZLIB
Compressed Data Format Specification Version 3.3 as part of RFC1950.
[RFC1950]
For each HEADERS compression instance, the initial state is
initialized using the following dictionary [UDELCOMPRESSION]:
const unsigned char http2_dictionary_txt[] = {
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0x73, 0x6f, 0x2d, 0x38, 0x38, 0x35, 0x39, 0x2d, \\ s o - 8 8 5 9 -
0x31, 0x2c, 0x75, 0x74, 0x66, 0x2d, 0x2c, 0x2a, \\ 1 - u t f - - -
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0x2c, 0x65, 0x6e, 0x71, 0x3d, 0x30, 0x2e \\ - e n q - 0 -
};
The entire contents of the name/value header block is compressed
using zlib. There is a single zlib stream for all name value pairs
in one direction on a connection. HTTP/2.0 uses a SYNC_FLUSH between
each compressed frame.
Implementation notes: the compression engine can be tuned to favor
speed or size. Optimizing for size increases memory use and CPU
consumption. Because header blocks are generally small, implementors
may want to reduce the window-size of the compression engine from the
default 15bits (a 32KB window) to more like 11bits (a 2KB window).
The exact setting is chosen by the compressor, the decompressor will
work with any setting.
4. HTTP Layering over HTTP/2.0
HTTP/2.0 is intended to be as compatible as possible with current
web-based applications. This means that, from the perspective of the
server business logic or application API, the features of HTTP are
unchanged. To achieve this, all of the application request and
response header semantics are preserved, although the syntax of
conveying those semantics has changed. Thus, the rules from the
HTTP/1.1 specification in RFC2616 [RFC2616] apply with the changes in
the sections below.
4.1. Connection Management
Clients SHOULD NOT open more than one HTTP/2.0 session to a given
origin [RFC6454] concurrently.
Note that it is possible for one HTTP/2.0 session to be finishing
(e.g. a GOAWAY message has been sent, but not all streams have
finished), while another HTTP/2.0 session is starting.
4.1.1. Use of GOAWAY
HTTP/2.0 provides a GOAWAY message which can be used when closing a
connection from either the client or server. Without a server GOAWAY
message, HTTP has a race condition where the client sends a request
(a new SYN_STREAM) just as the server is closing the connection, and
the client cannot know if the server received the stream or not. By
using the last-stream-id in the GOAWAY, servers can indicate to the
client if a request was processed or not.
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Note that some servers will choose to send the GOAWAY and immediately
terminate the connection without waiting for active streams to
finish. The client will be able to determine this because HTTP/2.0
streams are determinstically closed. This abrupt termination will
force the client to heuristically decide whether to retry the pending
requests. Clients always need to be capable of dealing with this
case because they must deal with accidental connection termination
cases, which are the same as the server never having sent a GOAWAY.
More sophisticated servers will use GOAWAY to implement a graceful
teardown. They will send the GOAWAY and provide some time for the
active streams to finish before terminating the connection.
If a HTTP/2.0 client closes the connection, it should also send a
GOAWAY message. This allows the server to know if any server-push
streams were received by the client.
If the endpoint closing the connection has not received any
SYN_STREAMs from the remote, the GOAWAY will contain a last-stream-id
of 0.
4.2. HTTP Request/Response
4.2.1. Request
The client initiates a request by sending a SYN_STREAM frame. For
requests which do not contain a body, the SYN_STREAM frame MUST set
the FLAG_FIN, indicating that the client intends to send no further
data on this stream. For requests which do contain a body, the
SYN_STREAM will not contain the FLAG_FIN, and the body will follow
the SYN_STREAM in a series of DATA frames. The last DATA frame will
set the FLAG_FIN to indicate the end of the body.
The SYN_STREAM Name/Value section will contain all of the HTTP
headers which are associated with an HTTP request. The header block
in HTTP/2.0 is mostly unchanged from today's HTTP header block, with
the following differences:
The first line of the request is unfolded into name/value pairs
like other HTTP headers and MUST be present:
":method" - the HTTP method for this request (e.g. "GET",
"POST", "HEAD", etc)
":path" - the url-path for this url with "/" prefixed. (See
RFC1738 [RFC1738]). For example, for
"http://www.google.com/search?q=dogs" the path would be
"/search?q=dogs".
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":version" - the HTTP version of this request (e.g.
"HTTP/1.1")
In addition, the following two name/value pairs must also be
present in every request:
":host" - the hostport (See RFC1738 [RFC1738]) portion of the
URL for this request (e.g. "www.google.com:1234"). This header
is the same as the HTTP 'Host' header.
":scheme" - the scheme portion of the URL for this request
(e.g. "https"))
Header names are all lowercase.
The Connection, Host, Keep-Alive, Proxy-Connection, and Transfer-
Encoding headers are not valid and MUST not be sent.
User-agents MUST support gzip compression. Regardless of the
Accept-Encoding sent by the user-agent, the server may always send
content encoded with gzip or deflate encoding.
If a server receives a request where the sum of the data frame
payload lengths does not equal the size of the Content-Length
header, the server MUST return a 400 (Bad Request) error.
POST-specific changes:
Although POSTs are inherently chunked, POST requests SHOULD
also be accompanied by a Content-Length header. There are two
reasons for this: First, it assists with upload progress meters
for an improved user experience. But second, we know from
early versions of HTTP/2.0 that failure to send a content
length header is incompatible with many existing HTTP server
implementations. Existing user-agents do not omit the Content-
Length header, and server implementations have come to depend
upon this.
The user-agent is free to prioritize requests as it sees fit. If the
user-agent cannot make progress without receiving a resource, it
should attempt to raise the priority of that resource. Resources
such as images, SHOULD generally use the lowest priority.
If a client sends a SYN_STREAM without all of the method, host, path,
scheme, and version headers, the server MUST reply with a HTTP 400
Bad Request reply.
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4.2.2. Response
The server responds to a client request with a SYN_REPLY frame.
Symmetric to the client's upload stream, server will send data after
the SYN_REPLY frame via a series of DATA frames, and the last data
frame will contain the FLAG_FIN to indicate successful end-of-stream.
If a response (like a 202 or 204 response) contains no body, the
SYN_REPLY frame may contain the FLAG_FIN flag to indicate no further
data will be sent on the stream.
The response status line is unfolded into name/value pairs like
other HTTP headers and must be present:
":status" - The HTTP response status code (e.g. "200" or "200
OK")
":version" - The HTTP response version (e.g. "HTTP/1.1")
All header names must be lowercase.
The Connection, Keep-Alive, Proxy-Connection, and Transfer-
Encoding headers are not valid and MUST not be sent.
Responses MAY be accompanied by a Content-Length header for
advisory purposes. (e.g. for UI progress meters)
If a client receives a response where the sum of the data frame
payload lengths does not equal the size of the Content-Length
header, the client MUST ignore the content length header.
If a client receives a SYN_REPLY without a status or without a
version header, the client must reply with a RST_STREAM frame
indicating a PROTOCOL ERROR.
4.2.3. Authentication
When a client sends a request to an origin server that requires
authentication, the server can reply with a "401 Unauthorized"
response, and include a WWW-Authenticate challenge header that
defines the authentication scheme to be used. The client then
retries the request with an Authorization header appropriate to the
specified authentication scheme.
There are four options for proxy authentication, Basic, Digest, NTLM
and Negotiate (SPNEGO). The first two options were defined in
RFC2617 [RFC2617], and are stateless. The second two options were
developed by Microsoft and specified in RFC4559 [RFC4559], and are
stateful; otherwise known as multi-round authentication, or
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connection authentication.
4.2.3.1. Stateless Authentication
Stateless Authentication over HTTP/2.0 is identical to how it is
performed over HTTP. If multiple HTTP/2.0 streams are concurrently
sent to a single server, each will authenticate independently,
similar to how two HTTP connections would independently authenticate
to a proxy server.
4.2.3.2. Stateful Authentication
Unfortunately, the stateful authentication mechanisms were
implemented and defined in a such a way that directly violates
RFC2617 - they do not include a "realm" as part of the request. This
is problematic in HTTP/2.0 because it makes it impossible for a
client to disambiguate two concurrent server authentication
challenges.
To deal with this case, HTTP/2.0 servers using Stateful
Authentication MUST implement one of two changes:
Servers can add a "realm=" header so that the two
authentication requests can be disambiguated and run concurrently.
Unfortunately, given how these mechanisms work, this is probably
not practical.
Upon sending the first stateful challenge response, the server
MUST buffer and defer all further frames which are not part of
completing the challenge until the challenge has completed.
Completing the authentication challenge may take multiple round
trips. Once the client receives a "401 Authenticate" response for
a stateful authentication type, it MUST stop sending new requests
to the server until the authentication has completed by receiving
a non-401 response on at least one stream.
4.3. Server Push Transactions
HTTP/2.0 enables a server to send multiple replies to a client for a
single request. The rationale for this feature is that sometimes a
server knows that it will need to send multiple resources in response
to a single request. Without server push features, the client must
first download the primary resource, then discover the secondary
resource(s), and request them. Pushing of resources avoids the
round-trip delay, but also creates a potential race where a server
can be pushing content which a user-agent is in the process of
requesting. The following mechanics attempt to prevent the race
condition while enabling the performance benefit.
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Browsers receiving a pushed response MUST validate that the server is
authorized to push the URL using the browser same-origin [RFC6454]
policy. For example, a HTTP/2.0 connection to www.foo.com is
generally not permitted to push a response for www.evil.com.
If the browser accepts a pushed response (e.g. it does not send a
RST_STREAM), the browser MUST attempt to cache the pushed response in
same way that it would cache any other response. This means
validating the response headers and inserting into the disk cache.
Because pushed responses have no request, they have no request
headers associated with them. At the framing layer, HTTP/2.0 pushed
streams contain an "associated-stream-id" which indicates the
requested stream for which the pushed stream is related. The pushed
stream inherits all of the headers from the associated-stream-id with
the exception of ":host", ":scheme", and ":path", which are provided
as part of the pushed response stream headers. The browser MUST
store these inherited and implied request headers with the cached
resource.
Implementation note: With server push, it is theoretically possible
for servers to push unreasonable amounts of content or resources to
the user-agent. Browsers MUST implement throttles to protect against
unreasonable push attacks.
4.3.1. Server implementation
When the server intends to push a resource to the user-agent, it
opens a new stream by sending a unidirectional SYN_STREAM. The
SYN_STREAM MUST include an Associated-To-Stream-ID, and MUST set the
FLAG_UNIDIRECTIONAL flag. The SYN_STREAM MUST include headers for
":scheme", ":host", ":path", which represent the URL for the resource
being pushed. Subsequent headers may follow in HEADERS frames. The
purpose of the association is so that the user-agent can
differentiate which request induced the pushed stream; without it, if
the user-agent had two tabs open to the same page, each pushing
unique content under a fixed URL, the user-agent would not be able to
differentiate the requests.
The Associated-To-Stream-ID must be the ID of an existing, open
stream. The reason for this restriction is to have a clear endpoint
for pushed content. If the user-agent requested a resource on stream
11, the server replies on stream 11. It can push any number of
additional streams to the client before sending a FLAG_FIN on stream
11. However, once the originating stream is closed no further push
streams may be associated with it. The pushed streams do not need to
be closed (FIN set) before the originating stream is closed, they
only need to be created before the originating stream closes.
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It is illegal for a server to push a resource with the Associated-To-
Stream-ID of 0.
To minimize race conditions with the client, the SYN_STREAM for the
pushed resources MUST be sent prior to sending any content which
could allow the client to discover the pushed resource and request
it.
The server MUST only push resources which would have been returned
from a GET request.
Note: If the server does not have all of the Name/Value Response
headers available at the time it issues the HEADERS frame for the
pushed resource, it may later use an additional HEADERS frame to
augment the name/value pairs to be associated with the pushed stream.
The subsequent HEADERS frame(s) must not contain a header for
':host', ':scheme', or ':path' (e.g. the server can't change the
identity of the resource to be pushed). The HEADERS frame must not
contain duplicate headers with a previously sent HEADERS frame. The
server must send a HEADERS frame including the scheme/host/port
headers before sending any data frames on the stream.
4.3.2. Client implementation
When fetching a resource the client has 3 possibilities:
the resource is not being pushed
the resource is being pushed, but the data has not yet arrived
the resource is being pushed, and the data has started to arrive
When a SYN_STREAM and HEADERS frame which contains an Associated-To-
Stream-ID is received, the client must not issue GET requests for the
resource in the pushed stream, and instead wait for the pushed stream
to arrive.
If a client receives a server push stream with stream-id 0, it MUST
issue a session error (Section 3.4.1) with the status code
PROTOCOL_ERROR.
When a client receives a SYN_STREAM from the server without a the
':host', ':scheme', and ':path' headers in the Name/Value section, it
MUST reply with a RST_STREAM with error code HTTP_PROTOCOL_ERROR.
To cancel individual server push streams, the client can issue a
stream error (Section 3.4.2) with error code CANCEL. Upon receipt,
the server MUST stop sending on this stream immediately (this is an
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Abrupt termination).
To cancel all server push streams related to a request, the client
may issue a stream error (Section 3.4.2) with error code CANCEL on
the associated-stream-id. By cancelling that stream, the server MUST
immediately stop sending frames for any streams with
in-association-to for the original stream.
If the server sends a HEADER frame containing duplicate headers with
a previous HEADERS frame for the same stream, the client must issue a
stream error (Section 3.4.2) with error code PROTOCOL ERROR.
If the server sends a HEADERS frame after sending a data frame for
the same stream, the client MAY ignore the HEADERS frame. Ignoring
the HEADERS frame after a data frame prevents handling of HTTP's
trailing headers
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.40).
5. Design Rationale and Notes
Authors' notes: The notes in this section have no bearing on the
HTTP/2.0 protocol as specified within this document, and none of
these notes should be considered authoritative about how the protocol
works. However, these notes may prove useful in future debates about
how to resolve protocol ambiguities or how to evolve the protocol
going forward. They may be removed before the final draft.
5.1. Separation of Framing Layer and Application Layer
Readers may note that this specification sometimes blends the framing
layer (Section 3) with requirements of a specific application - HTTP
(Section 4). This is reflected in the request/response nature of the
streams, the definition of the HEADERS and compression contexts which
are very similar to HTTP, and other areas as well.
This blending is intentional - the primary goal of this protocol is
to create a low-latency protocol for use with HTTP. Isolating the
two layers is convenient for description of the protocol and how it
relates to existing HTTP implementations. However, the ability to
reuse the HTTP/2.0 framing layer is a non goal.
5.2. Error handling - Framing Layer
Error handling at the HTTP/2.0 layer splits errors into two groups:
Those that affect an individual HTTP/2.0 stream, and those that do
not.
When an error is confined to a single stream, but general framing is
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in tact, HTTP/2.0 attempts to use the RST_STREAM as a mechanism to
invalidate the stream but move forward without aborting the
connection altogether.
For errors occuring outside of a single stream context, HTTP/2.0
assumes the entire session is hosed. In this case, the endpoint
detecting the error should initiate a connection close.
5.3. One Connection Per Domain
HTTP/2.0 attempts to use fewer connections than other protocols have
traditionally used. The rationale for this behavior is because it is
very difficult to provide a consistent level of service (e.g. TCP
slow-start), prioritization, or optimal compression when the client
is connecting to the server through multiple channels.
Through lab measurements, we have seen consistent latency benefits by
using fewer connections from the client. The overall number of
packets sent by HTTP/2.0 can be as much as 40% less than HTTP.
Handling large numbers of concurrent connections on the server also
does become a scalability problem, and HTTP/2.0 reduces this load.
The use of multiple connections is not without benefit, however.
Because HTTP/2.0 multiplexes multiple, independent streams onto a
single stream, it creates a potential for head-of-line blocking
problems at the transport level. In tests so far, the negative
effects of head-of-line blocking (especially in the presence of
packet loss) is outweighed by the benefits of compression and
prioritization.
5.4. Fixed vs Variable Length Fields
HTTP/2.0 favors use of fixed length 32bit fields in cases where
smaller, variable length encodings could have been used. To some,
this seems like a tragic waste of bandwidth. HTTP/2.0 choses the
simple encoding for speed and simplicity.
The goal of HTTP/2.0 is to reduce latency on the network. The
overhead of HTTP/2.0 frames is generally quite low. Each data frame
is only an 8 byte overhead for a 1452 byte payload (~0.6%). At the
time of this writing, bandwidth is already plentiful, and there is a
strong trend indicating that bandwidth will continue to increase.
With an average worldwide bandwidth of 1Mbps, and assuming that a
variable length encoding could reduce the overhead by 50%, the
latency saved by using a variable length encoding would be less than
100 nanoseconds. More interesting are the effects when the larger
encodings force a packet boundary, in which case a round-trip could
be induced. However, by addressing other aspects of HTTP/2.0 and TCP
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interactions, we believe this is completely mitigated.
5.5. Compression Context(s)
When isolating the compression contexts used for communicating with
multiple origins, we had a few choices to make. We could have
maintained a map (or list) of compression contexts usable for each
origin. The basic case is easy - each HEADERS frame would need to
identify the context to use for that frame. However, compression
contexts are not cheap, so the lifecycle of each context would need
to be bounded. For proxy servers, where we could churn through many
contexts, this would be a concern. We considered using a static set
of contexts, say 16 of them, which would bound the memory use. We
also considered dynamic contexts, which could be created on the fly,
and would need to be subsequently destroyed. All of these are
complicated, and ultimately we decided that such a mechanism creates
too many problems to solve.
Alternatively, we've chosen the simple approach, which is to simply
provide a flag for resetting the compression context. For the common
case (no proxy), this fine because most requests are to the same
origin and we never need to reset the context. For cases where we
are using two different origins over a single HTTP/2.0 session, we
simply reset the compression state between each transition.
5.6. Unidirectional streams
Many readers notice that unidirectional streams are both a bit
confusing in concept and also somewhat redundant. If the recipient
of a stream doesn't wish to send data on a stream, it could simply
send a SYN_REPLY with the FLAG_FIN bit set. The FLAG_UNIDIRECTIONAL
is, therefore, not necessary.
It is true that we don't need the UNIDIRECTIONAL markings. It is
added because it avoids the recipient of pushed streams from needing
to send a set of empty frames (e.g. the SYN_STREAM w/ FLAG_FIN) which
otherwise serve no purpose.
5.7. Data Compression
Generic compression of data portion of the streams (as opposed to
compression of the headers) without knowing the content of the stream
is redundant. There is no value in compressing a stream which is
already compressed. Because of this, HTTP/2.0 does allow data
compression to be optional. We included it because study of existing
websites shows that many sites are not using compression as they
should, and users suffer because of it. We wanted a mechanism where,
at the HTTP/2.0 layer, site administrators could simply force
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compression - it is better to compress twice than to not compress.
Overall, however, with this feature being optional and sometimes
redundant, it is unclear if it is useful at all. We will likely
remove it from the specification.
5.8. Server Push
A subtle but important point is that server push streams must be
declared before the associated stream is closed. The reason for this
is so that proxies have a lifetime for which they can discard
information about previous streams. If a pushed stream could
associate itself with an already-closed stream, then endpoints would
not have a specific lifecycle for when they could disavow knowledge
of the streams which went before.
6. Security Considerations
6.1. Use of Same-origin constraints
This specification uses the same-origin policy [RFC6454] in all cases
where verification of content is required.
6.2. HTTP Headers and HTTP/2.0 Headers
At the application level, HTTP uses name/value pairs in its headers.
Because HTTP/2.0 merges the existing HTTP headers with HTTP/2.0
headers, there is a possibility that some HTTP applications already
use a particular header name. To avoid any conflicts, all headers
introduced for layering HTTP over HTTP/2.0 are prefixed with ":". ":"
is not a valid sequence in HTTP header naming, preventing any
possible conflict.
6.3. Cross-Protocol Attacks
By utilizing TLS, we believe that HTTP/2.0 introduces no new cross-
protocol attacks. TLS encrypts the contents of all transmission
(except the handshake itself), making it difficult for attackers to
control the data which could be used in a cross-protocol attack.
6.4. Server Push Implicit Headers
Pushed resources do not have an associated request. In order for
existing HTTP cache control validations (such as the Vary header) to
work, however, all cached resources must have a set of request
headers. For this reason, browsers MUST be careful to inherit
request headers from the associated stream for the push. This
includes the 'Cookie' header.
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7. Privacy Considerations
7.1. Long Lived Connections
HTTP/2.0 aims to keep connections open longer between clients and
servers in order to reduce the latency when a user makes a request.
The maintenance of these connections over time could be used to
expose private information. For example, a user using a browser
hours after the previous user stopped using that browser may be able
to learn about what the previous user was doing. This is a problem
with HTTP in its current form as well, however the short lived
connections make it less of a risk.
7.2. SETTINGS frame
The HTTP/2.0 SETTINGS frame allows servers to store out-of-band
transmitted information about the communication between client and
server on the client. Although this is intended only to be used to
reduce latency, renegade servers could use it as a mechanism to store
identifying information about the client in future requests.
Clients implementing privacy modes, such as Google Chrome's
"incognito mode", may wish to disable client-persisted SETTINGS
storage.
Clients MUST clear persisted SETTINGS information when clearing the
cookies.
TODO: Put range maximums on each type of setting to limit
inappropriate uses.
8. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
9. Acknowledgements
This document includes substantial input from the following
individuals:
o Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa
Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam
Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay,
Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).
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o Gabriel Montenegro and Willy Tarreau (Upgrade mechanism)
o William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro,
Jitu Padhye, Roberto Peon, Rob Trace (Flow control principles)
o Mark Nottingham and Julian Reschke
10. Normative References
[ASCII] "US-ASCII. Coded Character Set - 7-Bit American
Standard Code for Information Interchange.
Standard ANSI X3.4-1986, ANSI, 1986.".
[HTTP-p1] Fielding, R. and J. Reschke, "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-21 (work in
progress), October 2012.
[HTTP-p2] Fielding, R. and J. Reschke, "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-21 (work in
progress), October 2012.
[RFC0793] Postel, J., "Transmission Control Protocol",
STD 7, RFC 793, September 1981.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738,
December 1994.
[RFC1950] Deutsch, L. and J. Gailly, "ZLIB Compressed Data
Format Specification version 3.3", RFC 1950,
May 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J.,
Lawrence, S., Leach, P., Luotonen, A., and L.
Stewart, "HTTP Authentication: Basic and Digest
Access Authentication", RFC 2617, June 1999.
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[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-
based Kerberos and NTLM HTTP Authentication in
Microsoft Windows", RFC 4559, June 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
[TLSNPN] Langley, A., "TLS Next Protocol Negotiation",
draft-agl-tls-nextprotoneg-01 (work in progress),
August 2010.
[UDELCOMPRESSION] Yang, F., Amer, P., and J. Leighton, "A
Methodology to Derive SPDY's Initial Dictionary
for Zlib Compression", .
Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-ietf-httpbis-http2-00
Changed title throughout.
Removed section on Incompatibilities with SPDY draft#2.
Changed INTERNAL_ERROR on GOAWAY to have a value of 2 .
Replaced abstract and introduction.
Added section on starting HTTP/2.0, including upgrade mechanism.
Removed unused references.
Added flow control principles (Section 3.5.1) based on .
A.2. Since draft-mbelshe-httpbis-spdy-00
Adopted as base for draft-ietf-httpbis-http2.
Updated authors/editors list.
Added status note.
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Authors' Addresses
Mike Belshe
Twist
EMail: mbelshe@chromium.org
Roberto Peon
Google, Inc
EMail: fenix@google.com
Martin Thomson (editor)
Microsoft
3210 Porter Drive
Palo Alto 94043
US
EMail: martin.thomson@skype.net
Alexey Melnikov (editor)
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
Belshe, et al. Expires July 26, 2013 [Page 50]